The present invention relates to a process for reducing the concentration of nitrogen oxides in the effluent gases from manufacturing plants.
Waste nitrogen oxides are released into the atmosphere both in the tail gases from plants manufacturing nitric acid and also in the residual gases produced by various plants using nitric acid as a reagent. There has been a constant search to minimize these waste products both for reasons of economy and to avoid pollution by toxic materials. Renewed emphasis to this search has been given by anti-pollution legislation in many countries. Such legislation characteristically sets limits to the weight of nitrogen oxides which may be released per ton of production, and generally excludes resorting to the simple palliative of diluting the exhaust gas with air.
Conventional plants manufacturing nitric acid from ammonia produce exhaust gases which contain from 1,000 to 2,000 cm.sup.3 of nitrogen oxides per cubic meter of effluent. In contrast to this, recent trends in legislation seek to impose a maximum permissible exhaust of nitrogen oxides equal to 1.5 Kg. per ton of nitric acid produced, corresponding in conventional nitric acid plants to about 200 cm.sup.3 of nitrogen oxides per cubic meter of the non-diluted tail gases.
The presence of nitrogen oxides in the effluent gases from nitric acid production can be understood in light of the well known reactions whereby the catalytic combustion of ammonia gives rise to nitric oxide, NO, which is inert to water. In the presence of oxygen, NO then forms nitrogen dioxide, which either by itself or in association with NO as dinitrogen trioxide then reacts with water on absorption columns according to the following reactions (1) and (2) to form nitric acid but also NO: EQU 3N.sub.2 O.sub.3 + H.sub.2 O .revreaction. 2HNO.sub.3 + 4NO (1) EQU 6NO.sub.2 or 3N.sub.2 O.sub.4 + 2H.sub.2 O .revreaction. 4HNO.sub.3 + 2NO (2)
unless specifically stated otherwise, in the present Specification and claims N.sub.2 O.sub.3 shall be understood to designate the equilibrium mixture of NO + NO.sub.2 .revreaction. N.sub.2 O.sub.3 resulting from the partial decomposition of one mol of N.sub.2 O.sub.3. Likewise N.sub.2 O.sub.4 shall be understood to designate the equilibrium mixture N.sub.2 O.sub.4 .revreaction. 2NO.sub.2 resulting from the partial decomposition of one mol of N.sub.2 O.sub.4 ; or conversely NO.sub.2 represents said equilibrium mixture resulting from the partial dimerization of one mol of NO.sub.2.
The regeneration of NO in these self-oxidationreduction reactions of N.sub.2 O.sub.3 and N.sub.2 O.sub.4 respectively makes the overall conversion to nitric acid a progressive process. The operation is made more difficult by the fact that decreasing the concentration of either NO or oxygen retards the rate of forming the desired NO.sub.2. As a result, it is extremely difficult to absorb the last traces of nitrogen oxides in the absorption system conventionally used in manufacturing nitric acid. Typically, the first two thirds of the absorption columns accomplish 98.5 percent of the operation but the remaining third is not sufficient to absorb completely the residual 1.5 percent and the tail gases contain at best 3 to 4 times as high a concentration of nitrogen oxides as is the present goal.
Attempts to lower the amounts of released nitrogen oxides by mere extension of the absorption system are fraught with difficult technical problems; also the additional installions would entail considerably increased investments.
In chemical plants utilizing nitric acid as a raw material, nitrogen oxide contaminants can be formed by the reaction of nitric acid with reducing agents. The products of such chemical reduction are sometimes limited to NO.sub.2 (and N.sub.2 O.sub.4) but the reduction may proceed further to NO. The effluents of these plants therefore have compositions similar to those found in nitric acid manufacture and can be subjected to the same treatments.
Among the prior methods proposed to limit the quantity of nitrogen oxide gases released into the atmosphere, there can be mentioned alkaline absorption which has the advantage of not generating more nitric oxide NO but also the disadvantage of not capturing any free NO. The reactions taking place in alkaline absorption are: EQU N.sub.2 O.sub.3 + Na.sub.2 O .revreaction. 2NaNO.sub.2 ( 3) EQU N.sub.2 O.sub.4 + Na.sub.2 O .revreaction. NaNO.sub.2 + NaNO.sub.3 ( 4)
the Na.sub.2 O, which can be introduced as NaOH or Na.sub.2 CO.sub.3 takes care of only NO.sub.2 or its N.sub.2 O.sub.3 compound with NO and permits any excess NO to escape, being inert to the alkali but capable of later reacting with atmospheric oxygen to form NO.sub.2.
The NO present in the gas to be treated usually is so dilute that its oxidation is very slow and the molar conversion of NO into NO.sub.2 is generally much less than the 50 percent required for complete absorption. Furthermore, the solution of nitrites and nitrates produced by this procedure cannot be released into natural waters but they are too dilute for recovery as commercially usable salts to be practical.
It has also been proposed to destroy the excess nitrogen oxides by burning them, as for example along with natural gas as a fuel. This has been found to be economically unfeasible, not because of the relatively small, say 0.2 percent, loss in nitric acid yield, but because of the considerable investments and running expenses which are involved in operating installations to dispose of the gases produced in the burner. Other methods using catalytic combustion require cumbersome use of expensive gases like hydrogen and methane as well as large investments for costly catalysts.